The present invention relates generally to current control and fault limiting in electrical power generation and transmission systems and more particularly to a solid-state circuit breaker apparatus having a current limiting characteristic and using a superconducting coil.
Continued growth of electrical power generation and transmission systems has resulted in higher levels of available fault currents and correspondingly greater demands on system protective components. It is not uncommon for utility systems today to be faced with fault currents higher than the momentary and interrupting capabilities of equipment installed. In a survey conducted in February 1979 by the EEI System Planning Committee on this subject it was determined that for the next 10-year period the estimated cost of circuit breaker replacement alone was in excess of 130 million dollars.
Developing higher current interrupting rated circuit breakers and retrofitting existing power systems therewith is prohibitively costly while restructing power system networks to reduce available fault currents destroys the advantages associated with tightly interconnected generation and transmission networks. Thus the desirability of fault current limiters compatible with and insertable into present power system networks is clearly seen, as long as such current limiters are simple, reliable, cost effective, and commercially viable.
A method of controlling currents in relatively small ac circuits was developed between 1940 and 1950 using a transductor. The transductor proved impractical for larger power systems, first because its dc circuit required more ampere turns than the ac circuit, second because the dc control and dc power supply requirement was substantial, and finally because the induced electromagnetic surges into the dc circuit from a dc fault presented serious mechanical and electrical problems.
To resolve some of the problems associated with the transductor, Darton proposed a superconducting current limiting circuit having a superconducting coil which controlled the dc bias current in a transductor connected in series with the ac power line, see K. C. Darton, "A New Power System Current Limiter," Electrical Review International, Vol. 202, No. 5, February 1978. Superconductivity eliminated the problems of the massive dc room temperature-winding, the high inherent loss in the dc circuit and, by appropriate design, the electrical and mechanical stresses arising from the surge problems. The concept of having available virtually unlimited dc ampere turns at negligible running cost to "flux flood" the ac cores appeared very promising. However, in the Darton system all components have to be rated to support practically full system line voltage and pass full system current. The rating of the transductor must be about equal to the system throughput power rating and it is therefore, a very bulky and costly component.